Single terminal electro-optical intruder detection device

ABSTRACT

An improved single terminal electric eye including a modulated luminous diode as a source of radiant energy and an alarmactivating circuit for detecting any variation, above or below a steady-state level, in reflected energy incident upon the transducing element of the electric eye. The alarm-activating circuit comprises a pair of averaging circuits connected in series and a comparator, connected to the outputs of the averaging circuits, which transmits an alarm-activating signal whenever the outputs of the averaging circuits differ. When the transducer is subjected to constant modulated illumination, the outputs of the averaging circuits are at the same steady-state level. When, however, the illumination increases or decreases, the outputs of the averaging circuits tend to follow at different rates, thereby providing an unbalanced input to the comparator whereby an alarm signal may be transmitted.

United States Patent Perlman 1 Feb. 22, 1972 [72] lnventor:

[73] Assignee:

David E. Perlman, Rochester, N.Y.

Detection Systems, Inc., East Rochester, N.Y.

[22] Filed: Sept. 18, 1969 [21] Appl. No.: 858,927

[52] 11.8. C1. ..340/258 B, 340/258 R, 250/217 SS [51] Int. Cl. .5081)17/10 [58] Field of Search ..340/258 B, 258 R, 261, 228 S, 340/228, 258C, 248 A; 250/217 SS, 221, 325/371, 377, 473, 474

[56] References Cited UNITED STATES PATENTS 2,730,896 1/ 1956 Boisblanc..340/261 UX 3,111,657 11/1963 Bagno ..340/258 3,167,739 1/1965 Girardet a1 340/258 X 3,198,952 8/1965 Benham ct a1. ..340/228 X 3,314,0664/1967 Schwartz et a1. 340/258 UX 3 ,465 ,3 39 9/1969 Mamet 340/248OTHER PUBLICATIONS F. F. Morehead, Jr., Light Emitting Semiconductors,Scientific American, May 1967, page 109, Group 250.

Primary Examiner--Donald .I Yusko Assistant ExaminerMichael SlobaskyAttorney-Warren W. Kurz [57] ABSTRACT An improved single terminalelectric eye including a modulated luminous diode as a source of radiantenergy and an alarm-activating circuit for detecting any variation,above or below a steady-state level, in reflected energy incident uponthe transducing element of the electric eye. The alarm-activatingcircuit comprises a pair of averaging circuits connected in series and acomparator, connected to the outputs of the averaging circuits, whichtransmits an alarm-activating signal whenever the outputs of theaveraging circuits differ. When the transducer is subjected to constantmodulated illumination, the outputs of the averaging circuits are at thesame steady-state level. When, however, the illumination increases ordecreases, the outputs of the averaging circuits tend to follow atdifferent rates, thereby providing an unbalanced input to the comparatorwhereby an alarm signal may be transmitted.

8 Claims, 5 Drawing Figures RECTIFIER AND FILTER VOLTAGE CURRENTREGULATOR osc'LLAmRl REGULATOR ALARM ACTFIzVATING SINGLE TERMINALELECTRO-OP'IICAL INTRUDER DETECTION DEVICE BACKGROUND OF THEINVENTION 1. Field ofthe Invention The present invention relates toimprovements in intruder detection devices of the electric eye variety.

2. Description of the Prior Art Conventional electric eyes employed forthe purpose of intruder detection generally comprise: (a) a transmitterof a beam of radiant energy; (b) a transducer which is electricallyresponsive to the energy provided by the transmitter and is capable ofproviding an electrical output proportional to the quantity of energyincident thereon; and (c) an alarm-activating circuit which iselectrically coupled with the transducer output and is energizable whenthe output changes in such a manner as to be indicative of a suddendecrease in energy incident upon the transducer.

The most common arrangement of the electric eye transmitting andtransducing elements is, of course, that arrangement wherein suchelements are positioned on opposite sides of an area to be protectedfrom intrusion, with the transducing element situated so as to interceptthe beam of energy provided by the transmitter. By this arrangement, thepresence of an intruder is readily detectable if, in some manner, hecauses an interruption in the beam, for such an interruption will causethe magnitude of energy incident on the transducer to suddenly decreasebelow the predetermined level at which the output of the transducer willenergize the alarm-activating circuit.

A less common arrangement of the electric eye transmitting andtransducing elements is one in which such elements are mounted adjacentto each other in a common housing and the energy beam is directed at asurface on the opposite side of the area to be protected, which surfaceis capable of reflecting a portion of the transmitted beam of energyback to the transducing element. Here again, intrusion is detectablewhen the reflected energy incident on the transducer is caused to dropbelow a predetermined level by the passage of the intruder through aportion of the beam. This arrangement is commonly known as a singleterminal electric eye.

Although the single terminal electric eye possesses many obviousadvantages over the more conventional dual terminal" arrangement, (e.g.,portability, relative ease ofinstallation, versatility, etc.), itsuffers a'substantial disadvantage in that it is readily foilable. Sinceconventional single terminal electric eyes, like the more conventionaldual terminal devices, operate on a signal dropout principle, thealarmactivating circuits associated therewith are not normallyenergizable until the reflected energy incident upon the transducingelement drops below a predetermined level. Thus, if the reflectivity ofthe intruder or his clothing is such as to reflect sufficient energyupon the transducing element as to cause its output to remain above thetriggering level of the alarm-activating circuit, his presence in theprotected area will remain undetected. Moreover, a wary intruder who isaware of the presence of an electric eye may readily escape detection bysimply short-circuiting the path of the energy beam by inserting amirror or the like in close proximity with the electric eye housing andthereby cause the energy at the transducing element to remain above thetriggering level. In this manner, the intruder may gain access to thatportion of the protected area to the rear ofthe mirror.

Another disadvantage of using conventional single terminal electric eyesis the necessity of using a reflecting surface in combination therewith.Without a reflecting surface, no energy from the transmitter will bereturned to the transducer and, therefore, the presence of an intruderwill not effect a decrease in energy on the transducer. Thus, ifreflecting sur face is unavailable in the area to be protected, aconventional single terminal electric eye cannot be utilized. Moreover,even when a reflecting surface is available, the electric eye must be,at all times, properly aligned therewith. Otherwise, insufficient energymay be reflected upon the transducer to detect an interruption of theenergy beam by the passage therethrough of an intruder.

Still another disadvantage of conventional electric eyes, both singleand dual terminal types, is that they commonly employ a heated filamentlamp as a source of radiant energy. In order to discriminate againstbackground radiation it is often necessary to modulate the output of theenergy source at a high frequency. Because of the comparatively slowtemperature decay of heated filaments, it is impractical to electricallymodulate the output of heated filament lamps at a high frequency. To doso, would result in a modulated signal having a small peak-to-peakamplitude. Thus, the alternative approach, mechanical modulation, isoften employed, and this tends to be costly due to the necessity ofemploying an electric motor for driving a reticle or the like in frontof the energy source. Moreover, such apparatus often requires asignificant amount of space which, in turn, requires the electric eyehousing to be of substantial size. Furthermore, insofar as heatedfilament lamps are concerned, such sources have the disadvantages ofhaving a relatively short life and of requiring relatively high powerinput. Such disadvantages obviously detract from the reliability of thesystem in which they are employed and add to the expense of operation,

SUMMARY OF THE INVENTION It is therefore a principal object of thepresent invention to provide a novel and improved single terminalelectric eye.

Another object of this invention is to provide a single terminalelectric eye which is more reliable than those presently available.

Still another object of this invention is to provide a single terminalelectric eye which is capable of properly operating with and without areflecting surface.

A further object of this invention is to provide a single terminalelectric eye which is capable of activating an alarm in response to thesensing of an increase, as well as a decrease, in reflected energyincident upon the transducing element.

A still further object of the present invention is to provide anelectronic control circuit for use in electric eye detection apparatuswhich is simple in construction and dependable in performance.

Yet another object of the present invention is to provide a compactsingle terminal electric eye having a source of radiant energy which maybe electrically modulated at high frequency for backgrounddiscrimination purposes.

In accordance with the present invention, it has been found that theforegoing objects can be accomplished by providing a single terminalelectric eye which generally comprises a luminous diode radiant energysource; an electronic oscillator for modulating the output of the diodeat high frequency; a transducer which is sensitive to energy emitted bythe diode and is arranged to receive reflected energy from objects whichhave been illuminated by the modulated energy provided by the diode; abackground discriminating amplifier, connected with the output of thetransducer and sharply tuned to pass a signal only of the modulationfrequency; and an alarm-activating circuit which is electrically coupledto the output of the tuned amplifier and is capable of producing analarm-activating signal whenever the output of the amplifier increasesor decreases a predetermined amount with respect to a steadystate level.The alarm-activating circuit comprises a pair of averaging circuits,electrically arranged in series and preferably having differing timeconstants, and a comparator circuit which is electrically coupled withthe outputs of the averaging circuits and is capable of producing analarm-activating signal whenever the outputs of the averaging circuitsdiffer.

A preferred embodiment of the invention has been chosen for the purposeof illustration and description and is shown in the accompanyingdrawings, brief descriptions of which are provided below.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic block diagram ofan electric eye circuit in accordance with a preferred embodiment of theinvention;

FIG. 2 is a schematic block diagram illustrating the circuit componentsof the alarm-activating circuit of FIG. 1;

FIG. 3 is series of graphs illustrating the outputs of circuitcomponents of the alarm-activating circuit as the input thereto varieswith time;

FIG. 4 is an electrical schematic diagram of the alarm-activatingcircuit of FIG. 2; and

FIG. 5 is a schematic block diagram illustrating circuit components ofthe alarm-activating circuit of FIG. 1 according to another embodiment.

DESCRIPTION OF PREFERRED EMBODIMENTS the radiation provided by thetransmitter 4 and capable of producing an electrical signal of amagnitude proportional to the magnitude of such radiation which isincident on the sensitive surface thereof, and an alarm-activatingcircuit 6 which is electrically coupled with the output of thetransducer 5.

Preferably, the transmitter comprises a luminous or lightemitting diode7 (e.g., gallium-arsenide or silicon carbide diodes) and a lens 8 whichserves to direct energy from the source outwardly from the housing 3 andthrough a space which is under surveillance by the electric eye.Luminous diodes are ideal for use in electric eyes because of theirrelatively small size and ability to be modulated at electronic speeds.Gallium arsenide luminous diodes are preferable because of their abilityto emit infrared radiation which is, of course, undetectable by thehuman eye, therefore, an intruder would be unable to determine thedirection in which the electric eye is aimed, even if he were aware ofits presence. Luminous diode 7 is energized and square wave modulated bya conventional circuit which includes a multivibrator oscillator 9, acurrent regulator 10 and a power amplifier 11 which is connected inseries with the cathode 12 ofthe luminous diode. Regulated voltage isprovided to the diode energizing and modulating circuit and to the anode13 of the luminous diode by a voltage regulator 14 which receives thedirect current output ofa rectifying and filtering circuit 15 which, inturn, is connected with a conventional alternating current source 16.

Transducer 5 preferably comprises silicon photodiode which is arrangedin the housing in such manner as to receive radiant energy which hasbeen transmitted by transmitter 4 and reflected by objects in the spaceunder surveillance. A lens serves to provide optical gain by condensingreflected energy upon the radiation sensitive surface of the transducer.The output of the transducer is then amplified by a conventionalpreamplifier 21 and the output thereof is then passed through a sharplytuned amplifier 22 which is tuned for the frequency of modulationprovided by oscillator 9. The output m of tuned amplifier 22 serves asthe input to the alarm-activating circuit 6, described hereinafter,which activates an alarm 25 when the output m varies in a predeterminedmanner above or below its long term or steady-state average value. Thealarm-activating circuit 6 is energized by positive voltage from asecond voltage regulator 27 connected with an output of the rectifyingand filtering circuit 15, and is provided with a negative referencevoltage from oscillator 9 through the rectifying and filtering circuit28, the negative voltage serving to enhance the output of circuit 6 wheninput m decreases from a preexisting low value.

In FIG. 2, the components of the alarm-activating circuit 6 are shownschematically. Output m of the tuned amplifier 22 is first fed through aconventional clamping circuit 30, commonly referred to as a DCrestorer," whereby the sine wave output of the tuned amplifier may beclamped to zero volts so that its full peak-to-peak amplitude can bedetected. Because of the normal distortion associated with clampingcircuits, the lower portion of the sine wave input will be clipped offby clamping circuit 30 and its output n will be as shown in the graph(a) of FIG. 3.

The clamping circuit output serves as the input n to a pair of averagingcircuits 31 and 32 which are connected in series. Averaging circuit 31serves to average its clipped sine wave input n and provide an output pwhich is proportional to the average value thereof. It should be notedthat, due to the inherently long time constant of averaging circuits ingeneral, output p of averaging circuit 31 will substantially lag anyrapid changes in input. Thus, as illustrated in graph (a) of FIG. 3wherein input n is shown to suddenly increase for a time period A andthen suddenly decrease, output p will always assume a level equal to theaverage value ofits input, but will assume such level at a much slowerrate than that at which the input changes. Output p is then fed to theinput of the second averaging circuit 32 which, again, averages itsinput and provides an output s proportional thereto; thus, circuit 32averages the average value of input n. Obviously, when input n isconstant, outputs p and s are identical. However, when input n changesto a new level, thereby causing output p to slowly change to a newlevel, output s will follow the change in output p but, again, at aslower rate. As best shown in graph (a) of FIG. 3, when input n isconstant, outputs p and s are identical; when input n changes to a newlevel, outputs p and s change accordingly, but at different rates.

Outputs p and s of averaging circuits 31 and 32, respectively, serve asthe inputs to a comparator circuit 33. Comparator 33 is capable ofproviding an output 1 whenever there is a difference in inputs exceedinga predetermined amount, output I having a magnitude related to theinitial magnitude of signal it and a duration substantially equal to theduration of the difference in inputs. Whenever the inputs to comparator33 are identical or sufficiently close as to be below the threshold atwhich an output signal may be generated, output I is zero. See graph (b)ofFIG. 3.

Whenever output 1 of comparator 33 is present, it is fed to anintegrating circuit 34 which serves as a means to discriminate againstfalse triggering of the alarm 25 due to rapid transients in the circuit.Integrator 34, having a relatively slow time constant, will graduallyaccumulate the output signal of comparator 33 and provide an output xproportional to the accumulated signal. When the accumulated outputexceeds a predetermined level, alarm 25 will be activated. Note, circuittransients may cause comparator 33 to transmit a signal of highmagnitude, but short duration. Such a signal would not cause triggeringof the alarm because of the slow response of the integrator.

To summarize the operation of the electric eye described hereinabovewith reference to its utility as an intruder-detecting device, theelectric eye housing is simply positioned such that a beam of modulatedenergy may be directed through the space to be protected and iselectrically energized from a conventional source. There is nonecessity, as in conventional devices, of directing the beam toward areflecting surface whereby energy may be returned to the transducingelement in order to establish a reference level. If there happens to bea reflecting surface (e.g., a wall) on the opposite side of theprotectedarea, the electric eye of the invention will adapt itself to use theenergy reflected thereby as reference level and will detect any increaseor decrease in such level effected by the intruder. Assuming theintruder absorbs more energy than he reflects, as is usually the case,then input n will decrease, causing outputs p and s of averagingcircuits to follow at different rates. Until outputs p and s arestabilized at a level indicative of the decreased energy incident ontransducer 5, comparator 33 will transmit a signal to integrator 34which, in turn, will accumulate such signal and activate the alarm 25when the accumulated signal surpasses a predetermined level. Assumingthe intruder reflects more of the energy provided by transmitter 4 thanhe reflects, thereby causing an increase of energy on transducer 5, theninput n will increase, causing outputs p and s of the averaging circuitsto follow at different rates. Again,

an alarm activating signal will be provided until outputs p and sstabilize at the new level. If there happens to be no reflecting surfaceat which the electric eye may be directed, then the electric eye of theinvention will adapt itself to use zero energy as the reference level.Whenever the transducer senses any increase in energy, as might becaused by the intruders passing through the beam and reflecting energyback to the transducer, the averaging circuits respond, at differentrates, to the increase in signal and an alarm-activating signal isgenerated by the comparator until the outputs of the averaging circuitsstabilize.

A preferred circuit for accomplishing the aforedescribed functions ofalarm-activating circuit 6 is illustrated in the electrical schematicdiagram of FIG. 4. As shown, the output m of tuned amplifier 22 is fedinto the clamping circuit 6, comprising capacitor C1 and diode D1, whichclamps the negativegoing peaks of the sinusoidal input to zero volts.The output of the clamping circuit is isolated from the followingcircuitry by the emitter-follower comprising transistor Q1 and its loadresistor R1. The output n of emitter follower Q1 is then fed into thefirst averaging circuit 31 comprising diode D2, and resistors R2 and R3,and capacitor C2. Resistor R2 provides a discharge path for C2, therebypermitting C2 to follow decreases in the input signal. The voltageaccumulated across capacitor C2 will be proportional to the averagevalue of the input to the averaging circuit. The output p of averagingcircuit 31 is then fed to the input of the second averaging circuit 32via the emitter of transistor Q2. The second averaging circuit 32comprises resistors R4 and R5 and capacitor C3. Voltage from a l2-voltsupply (voltage regulator 27) is accumulated across C3 which chargesthrough resistor R4 via transistor 02 and resistor R6. When the voltageacross capacitor C3 reaches the voltage across capacitor C2, transistorQ2 stops conducting and the outputs of averaging circuits 31 and 32 aresubstantially identical. The comparator circuit 33 comprises transistorQ2 (which serves the dual role of charging capacitor C3 when signal 12increases and also passes the presence of such increases on to theoutput I) transistors Q3 and Q4 and resistors R6, R7, and R9. Theintegrator 34 comprises resistor R8 and capacitor C4.

The elements of the alarm-activating circuit function in the followingmanner: when the voltages across C2 and C3 are substantially the same,thereby indicating a constant steadystate input from tuned amplifier 22,transistors Q2, Q3 and Q4 are cut off due to the absence of forward biasand the output it of the alarm-activating circuit is 6 volts. A negativevoltage is preferred because it provides for increased output swing whenthe input to the circuit is decreased from a preexisting low level. Whenthe input from tuned amplifier 22 decreases, the voltage acrosscapacitor C2 discharges proportionally through resistor R2, therebycausing the voltage across capacitor C3 to exceed that across C2. Whenthe voltage across C3 exceeds that across C2, an unbalance is created inthe circuit and transistor Q4 becomes forward biased and conducts untilcapacitor C3 discharges through resistor R5 to such an extent that thevoltages across C2 and C3 are again substantially equal. Conduction oftransistor Q4 causes a positive-going voltage to be developed acrossload resistor R9 which is then fed through resistor R8 to capacitor C4which integrates it until the accumulated charge exceeds the triggeringlevel of a relay drive circuit comprising alarm 25. When the input fromtuned amplifier 22 increases, the voltage across capacitor C2 increasesproportionally, thereby causing an unbalance with the voltage acrosscapacitor C3. When the voltage across R7 serves to reduce the gain oftransistor Q3 so that the overall sensitivity to signal increases doesnot exceed that of signal decreases. Transistor Q3 functions as aninverter so as to cause a voltage increase to appear across the outputload resistor R9. Thus, the integrating circuit receives a positivegoinginput, notwithstanding the direction of change in input to thealarm-activating circuit. Capacitor C5 serves to suppress unwanted brieftransients thereby acting as a prefllter for integrator 34.

In FIG. 5, there is schematically illustrated an alternate arrangementof the averaging circuits comprising the alarm-activating circuit 6. Asshown, the averaging circuits, 31 and 32' are arranged in parallel, ascontrasted with the series arrangement of the preferred embodiment. in aparallel arrangement, it is necessary that the time constants, TCl andTC2, of the averaging circuits be different, otherwise there would be nodifference in inputs to comparator 33 thereby preventing thetransmission of an alarm signal.

Although the single terminal electric eye of the present invention hasbeen described with particular reference to its application in intruderdetection, it should be understood that the invention has equal utilityin all fields wherein a change in condition may be detected by a changein reflected energy at the electric eye terminal. Moreover, it isapparent that the apparatus, and particularly the detailed circuitelements thereof, may be varied without departing from the spirit andscope of the invention as defined in the appended claims.

lclaim:

1. A single terminal photoelectric intruder detection device comprising:

a housing; means disposed on said housing for transmitting a beam ofelectromagnetic radiation through a region wherein intrusion is to bedetected, said transmitting means comprising a luminous diode;

means for electronically modulating the output of said luminous diode;

radiation-sensitive means disposed on said housing to receive saidradiation upon being reflected by objects in said beam, saidradiation-sensitive means comprising a photo diode which is responsiveto said radiation to provide an electrical output signal having anamplitude proportional to the intensity of radiation received thereby;and

means, operatively coupled with said electrical output signal, fortransmitting an alarm-activating signal in response to a predeterminedincrease or decrease in amplitude of said electrical output signaloccurring at a rate in excess of a predetermined value and lasting for apredetermined time.

2. A single terminal photoelectric intruder detection device comprisinga housing, a luminous diode disposed on said housing, electronic meansfor periodically energizing said luminous diode to cause modulatedradiant energy, at a predetermined frequency, to emanate therefrom,means for directing said energy through a space wherein intrusion is tobe detected, a photo diode disposed on said housing to be illuminated bysaid energy upon being reflected by objects in the space whereinintrusion is to be detected, said photo diode being responsive to saidenergy to produce an electrical output signal having an amplitudeproportional to the level of illumination received thereby, andcircuitry for activating an alarm in response to a predeterminedincrease or decrease in amplitude of said electrical output signal, saidincrease or decrease occurring at a rate in excess of a predeterminedvalue {and lasting for a predetermined time.

3. The invention according to claim 2 further comprising a tunedamplifier tuned for said predetermihed frequency and having an inputconnected with said electrical output signal.

4. The invention according to claim 3 wherein said alarmactivatingcircuitry comprises -a first averaging circuit having an input connectedwith said electrical output signal, a second averaging circuit having aninput connected with the output of said first averaging circuit, and acomparator circuit connected with the outputs ofsfaidfist and secondaveraging circuits for providing an alarm-activating signal in responseto a predetermined difference in amplitude of the outputs of said firstand second averaging circuits.

5. A single terminal photoelectric intruder detection device comprisinga housing, a luminous diode disposed on said housing, means forenergizing said luminous diode to cause radiant energy to emanatetherefrom, means for directing said energy through a space whereinintrusion is to be detected, a photo diode disposed on said housing tobe illuminated by said energy upon being reflected by objects in thespace wherein intrusion is to be detected, said photo diode beingresponsive to said energy to produce an electrical output signal havingan amplitude proportional to the level of illumination received thereby,and circuitry for activating an alarm in response to a predeterminedincrease or decrease in amplitude of said electrical output signal, saidincrease or decrease occurring at a rate in excess of a predeterminedvalue and lasting for a predetermined time, said alarm-activatingcircuitry comprising first and second averaging circuits connected inparallel, each of said averaging circuits having a time constantdifferent from that of the other and an input connected with saidelectrical output signal; and a comparator circuit operatively coupledwith the outputs of said averaging circuits and adapted to provide analarm activating signal when the difference in amplitude of therespective averaging circuit outputs exceeds a predetermined value.

6. Electric eye detection apparatus comprising means for transmitting abeam of radiant energy through a space to be monitored. transducingmeans responsive to said radiant energy and capable of providing anelectrical output signal proportional to the level of said energyincident thereon, said transducing means being arranged to receiveenergy from said beam which has been reflected by objects in said space,an alarm, and an alarm-actuating circuit comprising a first averagingcircuit operatively coupled with the output of said transducing meansoutput, a second averaging circuit having its input operatively coupledwith the output of said first averaging circuit, and a comparatorcircuit operatively coupled with the outputs of said first and secondaveraging circuits and adapted to provide an alarm activating outputsignal in response to a predetermined difference in output from saidaveraging circuits.

7. The invention according to claim 6 wherein said energytransmittingmeans comprises a luminous diode.

8. The invention according to claim 6 wherein said transducing meanscomprises a photo diode.

1. A single terminal photoelectric intruder detection device comprising:a housing; means disposed on said housing for transmitting a beam ofelectromagnetic radiation through a region wherein intrusion is to bedetected, said transmitting means comprising a luminous diode; means forelectronically modulating the output of said luminous diode;radiation-sensitive means disposed on said housing to receive saidradiation upon being reflected by objects in said beam, saidradiation-sensitive means comprising a photo diode which is responsiveto said radiation to provide an electrical output signal having anamplitude proportional to the intensity of radiation received thereby;and means, operatively coupled with said electrical output signal, fortransmitting an alarm-activating signal in response to a predeterminedincrease or decrease in amplitude of said electrical output signaloccurring at a rate in excess of a predetermined value and lasting for apredetermined time.
 2. A single terminal photoelectric intruderdetection device comprising a housing, a luminous diode disposed on saidhousing, electronic means for periodically energizing said luminousdiode to cause modulated radiant energy, at a predetermined frequency,to emanate therefrom, means for directing said energy through a spacewherein intrusion is to be detected, a photo diode disposed on saidhousing to be illuminated by said energy upon being reflected by objectsin the space wherein intrusion is to be detected, said photo diode beingresponsive to said energy to produce an electrical output signal havingan amplitude proportional to the level of illumination received thereby,and circuitry for activating an alarm in response to a predeterminedincrease or decrease in amplitude of said electrical output signal, saidincrease or decrease occurring at a rate in excess of a predeterminedvalue and lasting for a predetermined time.
 3. The invention accordingto claim 2 further comprising a tuned amplifier tuned for saidpredetermined frequency and having an input connected with saidelectrical output signal.
 4. The invention according to claim 3 whereinsaid alarm-activating circuitry comprises a first averaging circuithaving an input connected with said electrical output signal, a secondaveraging circuit having an input connected with the output of saidfirst averaging circuit, and a comparator circuit connected with theoutputs of said first and second averaging circuits for providing analarm-activating signal in response to a predetermined difference inamplitude of the outputs of said first and second averaging circuits. 5.A single terminal photoelectric intruder detection device comprising ahousing, a luminous diode disposed on said housing, means for energizingsaid luminous diode to cause radiant energy to emanate therefrom, meansfor directing said energy through a space wherein intrusion is to bedetected, a photo diode disposed on said housing to be illuminated bysaid energy upon being reflected by objects in the space whereinintrusion is to be detected, said photo diode being responsive to saidenergy to produce an electriCal output signal having an amplitudeproportional to the level of illumination received thereby, andcircuitry for activating an alarm in response to a predeterminedincrease or decrease in amplitude of said electrical output signal, saidincrease or decrease occurring at a rate in excess of a predeterminedvalue and lasting for a predetermined time, said alarm-activatingcircuitry comprising first and second averaging circuits connected inparallel, each of said averaging circuits having a time constantdifferent from that of the other and an input connected with saidelectrical output signal; and a comparator circuit operatively coupledwith the outputs of said averaging circuits and adapted to provide analarm activating signal when the difference in amplitude of therespective averaging circuit outputs exceeds a predetermined value. 6.Electric eye detection apparatus comprising means for transmitting abeam of radiant energy through a space to be monitored, transducingmeans responsive to said radiant energy and capable of providing anelectrical output signal proportional to the level of said energyincident thereon, said transducing means being arranged to receiveenergy from said beam which has been reflected by objects in said space,an alarm, and an alarm-actuating circuit comprising a first averagingcircuit operatively coupled with the output of said transducing meansoutput, a second averaging circuit having its input operatively coupledwith the output of said first averaging circuit, and a comparatorcircuit operatively coupled with the outputs of said first and secondaveraging circuits and adapted to provide an alarm activating outputsignal in response to a predetermined difference in output from saidaveraging circuits.
 7. The invention according to claim 6 wherein saidenergy-transmitting means comprises a luminous diode.
 8. The inventionaccording to claim 6 wherein said transducing means comprises a photodiode.